Radio direction finder



Feb. 22, 1949. R. H. HERRICK RADIO DIRECTION FINDER 9 Sheets-Sheet 1 Filed Oct. 4, 1943 INVENTOR. RQSWE LL H. HERRICK ATTORNEYS Feb. 22, 194-9. H, HERRlCK 2,462,18

RADIO DIRECTION FINDER Filed Oct. 4, 1943 9 Sheets-Shed 2 1 co w 0 9. N

' INVENTOR.

ROSWELL H. HERRICK ATTORNEYa R. H. HERRICK RADIO DIRECTION FINDER Feb. 22, 1949.

9 Sheets-Sheet 3 Filed Oct. 4, 1943 INVENTOR. ROSWELL H. HERFHOK ATTORNEYS R. H. HERRICK RADIO DIRECTION FINDER- Feb. 22, 1949.

9 Sheets-Sheet 4 Filed Oct. 4, 1945 mmw w in INVENTOR. ROSWELL H HE RRICK mmv mmm

ATTORNEYS Feb. 22, 1949. H HERRlCK RADIO DIRECTION FINDER 9 Sheets-Sheet 5 Filed Oct. 4, 1943 Rosweu. H. HERRICK ATTORNEYS Feb. 22, 1949. HERRlCK 2,462,188

RADIO DIRECTION FINDER IN VEN TOR.

ROSWELL H. HERRICK I ATTORNEYS Feb. 22, 1949. R. H. HERRICK 2,462,188

RADIO DIRECTION FINDER Filed Oct. 4, 1943 9 Sheets-Sheet 7 FIG. 8 FIG. 7 7

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9o INVENTOR.

ROSWELL H. HERRICK BY I 00 I W,$WAAK7,MM

ATTORNEYS Feb. 22, 1949.

Filed Oct. 4, 1943 ,R. H. HERRICK RADIO DIRECTION FINDER 9 Sheets-Sheet 8 FIG. l2

FIG. l3

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INVENTOR.

ROSWELL H. HERRICK ",of,M

ATTORNEYS Patented Feb. 22, 1949 lJNlTED s'rA'rss tartar orries Roswell H. Herrick, Oak Park, Ill., assignor to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application October 4, 1943, Serial No. 504,985

21-Claims. I The present invention relates .to radio direction finders and, more particularly, to a radio direction finder-suitable for operation in several of the higher radio frequency bands wherein amplitude modulated, frequency modulated, and continuous wave signals are to be received.

Heretofore various types of radio direction finders have been devised which range from the simple single loop antenna radio receiver to the more complicated systems such as the Bellinnitosi andthe Adco'cky The simple single vertical loop antenna has as its predominant disadvantage its susceptibility to distortion due to night efiectf Night eifect is the term applied to the distortion introduced in the response pattern of the antenna due to induced voltages appearing at the output of the antenna which result in a false indication of the null position. Vertically polarized-waves traveling parallel with the plane of-the loop antenna, irrespective of the angle of elevation of the direction of travel of such waves, produce maximum response at the loop antenna output terminals. Vertically polarized waves traveling toward the plane of the loop antenna, irrespective of the angle of elevation of the'direction of travel of such waves, produce no response at the loop antenna output terminals. A horizontally polarized wave traveling horizontally produces no response at the antenna output terminals. A horizontally polarized wave traveling'with an angle of elevation of the direction of travel of such waves produces maximum response when traveling toward the plane of the loop antenna. Such horizontally polarized waves traveling non-horizontally are produced by reflections from' the I-onosphere or Kennelly-Heaviside layer. At night'the reflection angles are greater, thus these wavespro- .duce a greaterresponse in theloop antenna and are termed night eifect. This response is maximum at the null position for vertically polarized waves travelling toward the plane of the loop and hence a different null position will be indicated.

"The single loop antenna has two null positions so that it is not possible to determine the direction of a transmitter but merely the plane in which a transmitter is located. In order to obviate this a non-directional antenna or sense antenna has been used in conjunction with a single loop antenna to produce a polar response diagram in the form of a cardioid. Thus a single null position is obtained but the cardioid is subject to the night efiects and hence the null position issubject to error.

was not as highly developed as at present, and

it had for its advantage the absence of the necessity of rotating the large massive loop antenna. Subsequentlythe Adcock system was developed in order to obviate the night efiect. The Adcock system with stationary antennas produced a clover-leaf response pattern which, however, introduces the disadvantage of providing four null indicationsso that a ninety. degree ambiguity might be introduced with respect to the plane in which a transmitter is operating. It thus becomes apparent that the problem of providing a radio direction finder is not a simple one, and that for a given set of conditions it is perhaps best to provide a particular solution, rather than to attempt to obtain an ideal universal direction finding system.

Accordingly, certain of these conditions will now be considered. In the present instance it was deemed desirable to provide a radio direction finder of the portable type which would operate over several relatively high radio frequency ranges and which could be used for direction finding of transmitters which may emit amplitude modulated, frequency modulated, or continuous wave signals. A primary requirement was the elimination of the night effect and the ninety degree ambiguity such as is inherent in the spaced loop system. The direction finding system should provide for audible identification of the transmitting station and a reliable, sensitive, error free visual indication of the plane in which the transmitter and receiver are located.

In accordance with the present invention these primary requirements were met by utilizing a pair of spaced parallel opposed loop antennas together with a sense antenna in the form of a loop antenna located midway between and parallel to the spaced parallel loop antennas, to eliminate the night effect. All of the loop antennas are maintained balanced with respect to ground. employed to provide a phase reversal when com- A minimum number of switches are bining the energies of the spaced parellel loop antennas with the sense loop antenna. Means are provided for controlling the ratio of the energies so combined so as to avoid overload efiects and mistuning of the receiver. It has been observed that this combination of antennas has an inherent advantage in that the ratio of the energies so combined is not critical and that variations in phase-relation may occur without introducing appreciable error in the null indi'cation.

In order to maintain the receiver apparatus and the operation thereof as simple as possible the sense loop antenna was provided with a resistive termination from which energy was obtained by an amplifying tube having an aperiodic input circuit. The output circuit of this tube is of the aperiodic "impedance-capacitance coupled type; thereby to avoid the requirement of a tuning capacitor together with a plurality of different impedances which otherwise would be required for each of the frequency bands over which the receiver is supposed to operate. The spaced loop antennas were connected through suitable reversing switches to a selected one of a plurality of tuned input transformers each of which was provided with shielding means for electrostatically balancing the primary winding with respect to ground. In order that there be no interaction between the selected transformer and those transformers not in use, each of the secondary windings of the unused transformers is normally short circuited. The input circuit of the amplifying tube associated with these input transformers is capacitively coupled to the plate circuit of the sense antenna amplifying tube. The sense antenna amplifying tube is provided with a manually operable gain control whereby the ratios of the energies combined may be controlled.

The present direction finder is arranged for operation over two high frequency radio frequency ranges, as, for example, the range of 2 to 4 megacycles and the range of 20 to 50 megacycles. It is desired to receive both narrow and wide band modulated signals such as amplitude modulated and frequency modulated signals in addition to continuous wave signals. At the higher frequency range of 20 to 50 megacycles a narrow response characteristic is desired for the reception of continuous wave and amplitude modulated signals, and a deviation of plus or minus 75 kilocycles is desired for the reception of frequency modulated signals. Accordingly, the present receiver utilizes a plurality of stages of radio frequency amplification of the impedance-capacitance tuned-input-impedance type. In order that the wide band modulation encounteredby frequency modulation might be properly amplified by a radio frequency amplifier, the lower frequency range impedances are given a broadened response characteristic by the introduction of resistors in shunt to the input impedance coils. Normally each of a plurality of tuned-input-impedance coils for the different frequency bands is short-circuited and switching means are provided for removing the short circuit from a desired one of the impedances and for connecting such impedance to the input electrode of its associated amplifier tube.

Because of the great difference between the two frequency ranges to be received the intermediate frequency amplifier is arranged to operate at two different intermediate frequencies such as 456 kilocycles and 5 megacycles. Each stage of intermediate frequency amplification therefore is provided with two intermediate frequency transformers, the primary and secondary windings of which are provided with adjustable trimmer capacitors arranged to operate parallel with afixed loading capacitor. In the frequency range of "2 to 4.5 megacycles it is desired to provide a broadened response of the intermediate frequency amplifier for a plus or minus 15 kilocycle deviation for the reception of frequency modulated signals. Each stage of amplification is provided with a pair of resistors adapted to be switched in circuit to provide this broadened response characteristic. In addition, at the higher frequency range of 20 to 50 megacycles a broadened response characteristic of plus or minus 75 kilocycle deviation is provided for frequency modulated signals. By provision of suitable switching arrangements these same resistors are again utilized to broaden the response of the intermediate frequency amplifier for 7 these signals. Whenever frequency modulated signals are being amplified by the intermediate frequency amplicuit the response broadening resistors.

fier the gain of the amplifier is increased by reducing the grid bias a predetermined amount. This change in bias is controlled by the same switching arrangement which connects into cir- The first detector and oscillator tube is provided with a plurality of tuned impedances which normally have one half oftheir windings short circuited so as to prevent any interaction between the impedance being used and those not being used.

Since the radio receiveris to operate over the frequency ranges of 2 to 4.5 megacycles and 20 to 50 megacycles, it has been found desirable to divide each range into two bands. Thus the radio frequency amplifier stages and the first detector oscillator are each provided with four transformers or impedances which are arranged to be switched into circuit by suitable switching means. This switching means also controls the selection of thetransformers to be used in the intermediate frequency amplifier.

The intermediate frequency amplifier is arranged to energize two separate channels. The one channel provides for the audible signals so that the station mightbe readily identified. The other channel converts signal energy into proportional direct current impulses to produce visual indication of the orientation of the directional antennas with respect to the transmitter, the signals of which are being received. This visual indication is obtained by a circuit arrangement wherein two capacitors each are charged by different direct currents obtained in accordance with different combinations of antenna voltages. A zero-center direct current instrument is connected across the two capacitors to give an indication of the resultant direct current voltage appearing across the capacitors. In order that the indicating instrument be deadbeat a resistor and an alternating current electrolytic capacitor are connected in shunt to the indicating instrument. This circuit arrangement therefore provides an indication which bears a relation to the reversal of phase of one of the antenna voltages, since the detector which is used to convert the radio frequency energies into proportional direct current energies is not effective to discriminate between phase relations of alternating currents.

In order to provide for the reception of continuous wave signals without requiring the use of an additional vacuum tube in the receiver, the detector in the visual indication circuit has a triode section which is used for the generation of oscillations, to provide audibility to the continuous wave signals. This oscillator is coupled to the input of the audible signal channel which, for continuous wave signals and amplitude modulated signals, comprises a stage of intermediate frequency amplification, a detector, and a stage of amplification. For the reception of frequency modulated signals the intermediate stage of amplification in this channel is converted-into a frequency modulation limiter. The detector'at thsame time is-converted into a frequency modulatiqn discriminator. These changes are accomplished by suitable circuit elements operated a switching means which control the frequencybands to be received by the receiver.

'It'is, therefore, an object of the present invention toprovide an improved radio direction finder wherein night effect has been eliminated. "Another object of the present invention is to provide an improved radio direction finder wherein night eflect hasbeen eliminated and which obviatesninety degree ambiguities.

Another object of the present invention is to provide an improved radio direction finder utili-Zinga pair of spaced loop antennas and a sense loop antenna wherein each of the antennas is maintained balanced with respect to ground.

'Another object of the present invention is to provide an improved radio direction finder utili'zing a plurality of loo antennas and a sense io'op'antenna which has a minimum of tuning apparatus.

" Another object of the invention is to provide an improved radio direction finder utilizing spaced loop antennas and a sense antenna which with a minimum of switches provides for the periodic reversal of phase when the energies of the antennas are combined.

It is another object of this invention to provide an improved radio direction finder whereby the plane in which the radio transmitter and the direction finder are located is indicated while the directional antennas are positioned so as to be influenced to the maximum extent by the waves of the radio transmitter.

A further object of invention is to provide an improved radio direction finder in which the energies of several directional antennas are combined and, in accordance with the combined energies, a visual response indication is provided which is substantially unaffected by minor phase variations of the energies so combined.

A further object of invention is to provide an improved radio direction finder which produces a visual response indication of sharply defined nulls.

A still further object of invention is to provide an improved radio receiver for operation at several of the higher radio frequency ranges.

A still further object of invention is to provlde an improved superheterodyne radio receiver for operation at the higher radio frequency ranges to receive amplitude modulated, frequency modulated, and continuous wave signals.

A still further object of invention is to pro-- vide in a' radio receiver an intermediate frequency amplifier selectively operable at two different intermediate frequencies.

Astill further object of invention is to provide in a superheterodyne radio receiver an intermediate frequency amplifier operable at two difierent intermediate frequencies with a cominon means for broadening the response characteristic of the amplifier at either of the intermediate frequencies.

'A still further object of invention is to pro- Vide in a radio receiver an intermediate frequency amplifier for the reception of narrow band andwide band modulated signals with means for increasing the gain of said amplifier for the reception of the wide band modulated signals.

Another and further object of the present invention is to provide a circuit for comparing direct curre ntswherein-a direct current instru mentisda-mped soils to be deadbeat.

. Otherand further objects of the present invention-will become .more readily apparent by referenceto the. following description taken in connection with the accompanying drawings wherein Figs'.1 to 6 comprise a circuit diagram of the radio direction finder comprising the present invention; Figs. 7 to 15-are graphical representations and curves to'explain the operation of'the' present invention; and Fig. 16is achart showingcertain switching operations performed in accordance with different receivin conditions.

.Referring more'particularly to Figs. 1 to 6, it will be'seenthataplurality of directional antennas has been provided. A pair of loop antennasI-UI and I02 are arranged in spaced parallel relation and the'connections thereto are so arranged that the voltages in the antennas are opposed so that a'resultant'voltage is obtained at theroiitput terminals of these two antennas. A third directional loopantenna I 03 is positioned midwaybetween and parallel'to'the pair of loop antennas IOI and I02. The loop antenna I03 serves as a sense antenna .so'that energy therefrom might be combined with the resultantenergy obtained from the pair of spaced opposed antennas; whereby a response may be obtained by the radio receiver which, translated by an indicating circuit, will show the null position of the directional antennas. The manner inwhich the voltages are obtained and the resultant indication is obtained will subsequently be explained after: the radio receiver has been described in .detail.

:In order to simplify'the radio receiver apparatus and at 'thesame time to obviate as far as possible the introduction of unbalances in any of the antennasawith respect to ground, a minimum of switches and tuning apparatus is employed. The sense antenna I03 is'provided with a resistive termination comprising the resistor network composed of thexresistors I04 to I01. The midpoint of this 'resistor network'is connected to ground. A portion-10f the voltage appearing across the resistivetermination of the loop antenna I03 is applied to'a vacuum tube I09 by a direct connec-v tion between the control grid thereof with one terminal of the resistor I06 and bya coupling capacitor108 connected between the cathode of the'vacuum tube I09 and the intermediate terminal of the resistor'network. It therefore becomes apparent that the input to the vacuum tube roars aperiodic .and that for all frequency ranges and tuning bands no individual impedance transformers or tuning apparatus is employed.

Thespaced parallel loop antennas IllI and I02 are connected together so as to produce a'resultant voltage across a pair of conductors H9 and I20 which are connected to certain switch-com tacts on a motor driven switch IIO. This switch is provided with a plurality of make and break contactsl'II to H6. The contacts III to H4 are 50 arranged with respect to the loop antenna conductors H9 and I20 as to comprise a double pole double throw switch, whereby the phase of the spaced loopantenna energy may be reversed. The remaining switch contacts H5 and III; are arranged to interconnect a pair of conductors '53I with conductors 532 and 5337to provide withrespect to the operationof the double pole double throw switch portion of the switch H0, concomitant operation corresponding to single pole double throw switch operation. This latter switching operation interconnects a direct current indicating circuit with that-portion of the receiver which produces direct currents proportional to the combined energies fed to the input of the radio receiver. The motor driven switch H is provided with a plurality of cams II1 so that the switch contacts III and H3 are made simultaneously while the switch contacts I I2 and I I4 are opened, and vice versa. The switch contacts H5 and H6 are arranged to complete a circuit to one of the conductors 532 or 533 subsequent to the operation of the previously mentioned switch contacts and prior to the subsequent operation of the contacts. This difference in timing is provided so as to insure that there will be no error in the value of the direct current voltages applied to the indicating circuit.

The cams I I1 are driven by a suitable direct current motor H8. The switch contacts H2 and H3 are connected to aconductor I30 which joins the lower contact of eachof a plurality of switch contacts SIb, S21), S32) and 84b. The switch contacts I I I and H4 are connected by the conductor I40 to the lower contacts of a plurality of switch contacts Sia, S2a, S30. and 84a. A plurality of switches SI s2, S3 and S4, each composed of pairs of switch contacts SIa, SIb, etc., are arranged for selective connection to the primary windings of a plurality of different input transformers I2I to I24, respectively. Two transformers I2I and I22 are provided for the two bands which comprise the first range of frequencies from 2 to 4.5

megacycles, and two other transformers I23 and.

I24 are provided for the two bands in the other range of frequency from to 50 megacycles.

Each of the transformers I2I to I24 is balanced with respect 'to ground by the provision of an electrostatic shield interposed between the primary and secondary windings.

The switch I I0 which interconnects the spaced loop antennas IOI and I02 with the balanced to ground input radio frequency transformer is constructed so as to be symmetrical in all respects to avoid introducing any unbalances in the antenna circuit. While the switch has been used between the spaced loop antennas and the radio frequency input transformers in order to reverse the phase of one of the energies being combined in the first stage of amplification of the radio rceiver, it of course will be appreciated by those skilled in the art that the switch migh have been arranged to reverse the phase of the energy supplied by the sense loop antenna I03, in which case the switch I I0 would interconnect the sense loop antenna I03 with the input to the vacuum tube I09.

One terminal of each of the secondary windings' of the transformers I2I to I24 is connected to a conductor I25 which, in turn, is connected to the grounded terminal I26 of a main tuning capacitor I3I and a Vernier tuning capacitor I32. The other terminals of the secondary windings of the transformers I2I to I24 are connected respectively to switches S5 to S8 which normally are arranged to short circuit the secondary windings of these transformers. By suitable switch operating apparatus any selected one of these switches may be actuated so as to cause the selected transformer terminal to be connected to the conductor I21 which is connected to the grid electrodeof a vacuum tube I28. The cathode of the vacuum tube I28 is connected through a fixed biasing resistor I42 and an adjustable biasing resistor I43 provided with an adjustable contact I44 connected to ground. .The common connection between the resistors I42 and I43'is also con- I02. i odically reversed by the operation of the switch nected to the conductor I51.

nected to a conductor 206. The cathode ofth vacuum tube I 28 is connected to a by-pass capac itor I29 which, in turn, is connected to ground. The output circuit or"v the vacuum tube I28 includes a choke coil I-5I and a voltage drop resistor I46 which, in turn, is connected to a conductor I34 which serves to supply a high potential for the anodes and the screen grids of the various vacuum tubes. The common juncture between the resistor I46 and the choke coil I5I is connected to a bypass capacitor I41 which is grounded. This common juncture is also connected to a current drop resistor I48 which is connected to the screen grid of the vacuum tube I28. The screen grid of the vacuum tube I28 is connected to a grounded bypass capacitor I49. The vacuum tube I20 therefore is provided with an input circuit which may be tuned so as to resonate at a desired frequency within any one of the four bands of frequencies covered by the input transformers I2I to I24. The vacuum tube I28 by means of the transformers I2I to I24 receives the resultant energy obtained from the opposed spaced parallel loops IOI and This energy, the phase of which is peri- I I0, is combined with energy received from the vacuum tube I09.

The vacuum tube I09 is provided with an output circuit which includes an anode choke coil I33 connected between the high voltage conductor I34 and the anode of the vacuum tube I08. The anode of the vacuum tube I09 is capacitively coupled to the input circuit of vacuum tube I28 by a coupling capacitor I4I connected to the conductor I21. Suitable voltage is supplied to the screen grid electrode of the vacuum tube I09 by means of a voltage drop resistor I35 connected between the conductor I34 and the screen grid. The screen grid of the vacuum tube I09 is connected to a grounded by-pass capacitor I36. Because the plate or anode of the vacuum tube I09 is capacitively coupled to the tuned input circuit of the vacuum tube I28, the vacuum tube I09 in fact utilizes this tuned circuit as its plate load and, hence, the tuned input circuit of the vacuum tube I28 serves as a mixing circuit for the voltages of the spaced parallel opposed loop antennas and the voltage component obtained from the sense loop antenna.

The anode of the vacuum tube I28 is coupled by a capacitor I58 to the grid of the next radio frequency amplifying tube 20I which is con- The conductor I51 is connected to the upper contact of each of a plurality of switches $9, $10, SH and SI2 which normally are arranged to short circuit a plurality of tuned impedances I52 to I55, each of which is provided with an auxiliary tuning capacitor I62 to I65, respectively. The auxiliary or trimmer tuning capacitors I62 to I65 each operate in parallel to the main tuning capacitor 202 which is connected between the grid of the vacuum tube 20I and a ground connection 204 which is also connected to a conductor I56. The conductor 205,; and the adjustablabiasing resistor I43 to; ground. It thus becomes apparent thatv the vacuum tube I 28 is provided with a predetermined self bias from the. resistor I42, and similarly the vacuum tube 2!]! is providedwith a predetermined self bias from the resistor 2G5, and these biases are augmented by the common bias provided by the drop through the resistor I43. The adjustable contact I44. on the resistor I43 is arranged to be actuated from the operating panel of the radio receiver so as to comprise a manually operable gain or sensitivity control for the radio frequency amplifier. The output circuit of the vacuum tube 2!!! includes a choke coil or impedance 2|! and a voltage drop resistor 208 connected between the anode and the high voltage conductor I34. The resistor 203 is provided with a grounded by-pass capacitor 22!. A lesser voltageis applied to the screen grid of the. vacuum tube 2:1 I by means of a voltage drop resistor 20.1. connected between, the. screen grid of the vacuum tube 20! and the common juncture of the resistor 209 and the choke coil 21!. The screen grid of the vacuum tube 29! is by-passed to ground through a by-pass capacitor 208. The output of thevacuum tube, 2IlI is coupled bya capacitor 242 to the input electrode of the next vacuum tube. 39.! by means of a conductor 24!.

The grid of the vacuum tube 3!]! is connected to a. tuning capacitor 239, the rotor of; which is grounded. The conductor 24! is connected to the upper contact of each of a plurality of. switches SI3 to SIB arranged for cooperation with a plurality of tuned impedances 23! to 234. The switches SIS and SIB. are arranged so that the tuned impedances when not in use are short circuited. One terminal of each of the tuned impedances 23! to 234 is connected to the ground. Auxiliary tuning or trimmer capacitors 235 to 238 are arranged in parallel to the tuned impedances 23! to 234. The lower frequency tuned impedances 23!v and 232 are provided with shunt resistors 228 and 223m order that their response characteristic might be broadened sufficiently for the transmission of wide band or frequency modulated signals in those frequency bands.

The vacuum tube 30! serves as a combined oscillator and first detector and, accordingly, an oscillator circuit is provided which is connected to the conductors 25! and 2 52 which are joined to the grid and cathode of this vacuum tube. The grid which is connected to the conductor 25I is connected through a resistor 249 to ground and is capacitively coupled to oscillation circuit by a capacitor 248 connected to the upper contacts of switches SIG and S22. These upper contacts of the switches SIS and S22 are coupled by a padding capacitor 243 to the upper contacts of a pair of switches SI! and sit and to one terminal of a grounded tuning capacitor 246 A plurality of impedances 2I2 to 2IQ are arranged for cooperation with a plurality of adjustable trimmer capacitors 222 to 225. The impedances 2I2 and 2I3 are joined together to form a series circuit across thecapacitor 222. The common juncture of the impedances 212 and 2I3, is connected to one terminal of a breakm ake switch S2i. The switch S2! is arranged so as to normally short circuit one of theseimpedances, such as the choke coil or impedance 2I2. One terminal of the other impedance or choke 2I3 is connected through a padding capacitor 226 to one of the contacts of a switch SI's. The remaining choke impedances 2.! 4 to 219 are similarly grouped together so that the choke impedanoe s 10 2M. and 215. are connected across the trimmer capacitor 223, the choke impedances 2H5 and 2!! are connected across the capacitor 224, and choke impedances 2J8 and 2I9 are connected across the capacitor 225. Similarly arranged switches S22, S23 and S2 1 are arranged so as to normally short circuit the choke impedances 2J4, 2H5 and 2I8. One terminal of the choke impedance 295 is connected through a padding capacitor 22'! tonne contact of the switch SI8. Each of the circuits therefore comprises a tuned oscillating circuit which may be placed in operation by the simultaneous operation of its two associated switches. The tuning capacitor 246 is common to all of these circuits and is arranged for adjustment by a unitary drive means interconnecting the tuning capacitors 239, 222 and I3I. The vacuum tube 301 is provided with a screen grid which is connected through resistors 393 and 395 to the anode voltage conductor I34. This grid is by-passed to ground b-ya grounded capacitor 3532. The voltage supplied to the screen grid is stabilized by the connection of a voltage regulator tube 304 between ground and the common juncture of the resistors 303and3l15.

The output of the vacuum tube 30! is connected to a multi-stage intermediate frequency amplifier which is arranged to operate at either of two selected intermediate frequencies. dependent upon which of the frequency ranges is being utilizedin the radio frequency amplifier.

The anode of the vacuum tube SM is connected through a switch S25 and a primary winding 32I of a transformer 329. and a voltage drop resistor M6 to the anode voltage conductor I34. The juncture of the resistor 3Ifi with the voltage conductor I34 is connected to a grounded bypass capacitor 3I8. The other terminal of the resistor 3I6 is connected to a grounded by-pass capacitor 3| 'i The transformer 32!] has a primary winding 32! and a secondary winding 322 which recpectively are provided with shunt trimmer capacitors 3,23 and 324. The trimmer capacitor 323 is arranged for operation in parallel with a fixed loading capacitor 306 to tune the primary winding 32!. The secondary winding of the transformer 32B is connected to ground and to one contact of a switch S26. The switch S26 is connected to the input grid electrode of a vacuum tube, 326. The trimmer capacitor 324 of the secondary winding 322 of the transformer 320 is arranged to operate in parallel with a fixed loading capacitor 3I9 to tune the transformer secondary winding 322. The switches S25 and S25 normally are in the position shown so that the transformer 323 is efiective for the operation of the intermediate frequency amplifier at a frequency of 456 kilocycles. This frequently is used when reception is had in the frequency range of 2 to 4,5 megacycles. When the reception is in the frequency range of 20 to 50 megacycles the switches S25 and S26 are actuated to connect into circuit another intermediate frequency transformer 3!!) which is arranged to operate at an intermediate frequency of 5000 kilocycles. The intermediate frequency transformer 3H3 is provided with a primary winding 38 i and a secondary winding 3!2 which are connected in parallel with trimmer capacitors 3I3 and 3M respectively. The capacitors 3I3 and 3! are each arranged to operate in parallel with loading capacitors 306 and 359 respectively to tune the primary and the secondary windings of the transformer 3H3. Th cathode of the vacuum tube 326 is connected through a voltage drop resistor. 328 to a conductor 1 1 329. The resistor 328 is provided with a grounded by-pass capacitor 321.

When the intermediate frequency transformer 320 is connected as shown the response characteristic thereof is not sufficiently broad for proper transmission of wide band or frequency modulated signals. In order to broaden the response characteristic of this transformer a resistor 3l5 is connected in shunt to the primary winding 32I by a switch S44, and another resistor 325 is connected in shunt to the secondary winding 322 by a switch S45. When the other intermediate frequency transformer 310 is used the response characteristic thereof adequately passes wide band modulated signals having a deviation of plus or minus 15 kilocycles. When, however, a deviation of plus or minus '75 kilocycles is desired, it is necessary to broaden the response characteristic of this transformer am. To accomplish this the resistors 3l5 and 325 are connected by the switches S44 and S45 in parallel with the primary and secondary windings respectively of the intermediate frequency transformer 3H Thus it becomes apparent that a single pair of resistors associated with a stage of intermediate frequency amplification serves to broaden the frequency response of either of two intermediate frequency transformers. The effect of the resistors 3I5 and 325, of course, is much greater on the intermediate frequency transformer 3I0 which operates at the higher intermediate frequency of 5000 kilocycles, and hence there is provided a broadened response characteristic for the transmission of modulation signals having a deviation of plus or minus '15 kilocycles.

The anode of the vacuum tube 326 is normally connected through a switch S21 and the primary winding 34! of an intermediate frequency transformer 340 and the voltage drop resistor 338 to the anode voltage conductor I34. The juncture of the resistor 338 with the voltage conductor I34 is connected to a grounded by-pass capacitor 351. The other terminal of the resistor 338 is connected to a grounded by-pass capacitor 345. The juncture between the resistor 338 and the primary winding 34I of the transformer 340 is connected through a voltage drop resistor 331 to a screen grid of the vacuum tube 326 which is connected to a grounded by-pass capacitor 339. The intermediate frequency transformer 340 has its primary winding 34! and its secondary winding 342 tuned by trimmer capacitors 343 and 344, respectively, connected in parallel with fixed capacitors 335 and 336.

Another intermediate frequency transformer 330 is provided with a primary winding 33I and a secondary winding 332 tuned by trimmer capacitors 333 and 334, respectively, in parallel with the fixed capacitors 335 and 333. The transformer 330 is arranged to be switched into operation by the actuation of switches S21 and S28. The

switch S28 is connected to a conductor 348 which is connected to the grid or input electrode of a vacuum tube 426. One terminal of each of the secondary windings 332 and 342 is connected to grounded conductor 349 which is connected to one terminal of a capacitor 421, the other terminal of which is connected to the cathode of the vacuum tube 426. A pair of resistors 346 and 341 are arranged to be connected by a pair of switches S46 and S41 in parallel with the primary and secondary windings of either of the transformers 330 or 340 in order to broaden the response characteristic thereof.

The vacuum tube 428 has its cathode connected 12 through a voltage drop resistor 428 to a conductor 329. The anode of thevacuum tube 426 is normally connected through a switch S29 to the primary winding 44! of the transformer 440 and through a resistor 438 to the voltage conductor :34, The common juncture between the resistor 438 and the primary winding I of the intermediate transformer 440 is connected to a resistor 431 which is connected to the screen grid of the vacuum tube 426., The screen grid of the vacuum tube 426 is connected to a grounded bypass capacitor 439. The primary winding I of the transformer 440 is arranged to be tuned by a trimmer capacitor 443 which operates in parallel with a fixed loading capacitor 435. The secondary winding 442 of the intermediate frequency transformer is tuned by a trimmer capacitor 444 arranged to operate in parallel with a fixed loading capacitor 436. The intermediate frequency transformer 440 is normally connected in circuit by the switches S29 and S30 to provide operation at the lower of the two intermediate frequencies.

An intermediate frequency transformer 430 is provided for operation at. the higher intermediate frequencies. The primary winding 43! of the transformer 430 is tuned by a trimmer capacitor 433 and operating in parallel with a fixed loading capacitor 435. The secondary winding 432 of this transformer is tuned by a trimmer capacitor 434 operating in parallel with a fixed loading capacitor 436. In order that either of the intermediate transformers 430 or 440 may have their response characteristic broadened, there is provided a pair of resistors 446 and 441 which are arranged to be connected in parallel to the primary and secondary windings, respectively, by a pair of switches S48 and S49. The secondary windings of the intermediate frequency transformers 430 and. 440 have the one terminal grounded and the other terminal arranged to be connected by operation of the switch S30 to the grid or input electrode of the vacuum tube 45!. The grid of the vacuum tube 45I is connected to a capacitor 48450 that a portion of the energy appearing across the input circuit of the vacuum tube 45I may be transmitted to another signal channel energized from a conductor 483. The cathode of the vacuum tube 45l is connected through a biasing resistor 453 to the conductor 329. The resistor 453 is by-passed to ground by a capacitor 452 connected to the cathode of the vacuum tube 45L tends to a stage of intermediate frequency iamplification energized from the conductor 483. The conductor 329 is connected through a resistor 454 to ground. Thus the resistors 328, 428 and 453 together with the resistor 454operate to provide a certain predetermined bias to the intermediate frequency amplifier thus far described. For the reception of certain signals, such as the frequency modulated signals, it is desirable to increase the gain of the intermediate frequency amplifier by a predetermined amount, and to thus change the bias there is provided a switch S52 which is arranged to short circuit the voltage drop resistor 454, thereby to increase the gain of the amplifier.

The visual indication channel includes two intermediate frequency transformers 460 and 410. The anode of the last amplifier tube of the previously described intermediate frequency amplifier has its anode normally connected through the switch S3] to the primary winding! of the intermediate transformer 410 and through a The conductor 329 also ex-- resistor: 45ato the high" voltage conductor I34. The juncture of the resistor 458- with the high voltage: conductor is'connected to a grounded bypass capacitor. 459. The" other terminal: of the resistor 458 is connected to a grounded by-pass capacitor 415. The primary winding 41 l' of the transformer 4-10 is arranged to be tuned by a trimmer capacitor 4'13 operating in parallel with afixed loading capacitor 465. The response characteristic of the intermediate frequency transformer isbroadened slightly by a connection'of the resistor 455 in parallel with the fixed capacitor 465. Voltage is supplied to the screen gridof the vacuum tube 45f through a voltage drop resistor 456 and the screen grid is connected to agrounded by-pass capacitor 451. The secondary winding 412 of the transformer 41B" is tuned by a trimmer capacitor 414 operating in paraller with a fixed loading capacitor 466 connected between the diode elements of a vacuum tube 418; The secondary winding 4-12 is normally' connected by the switches S32 and S33 tothese-diode elements. When the switches S3! to S33 are actuated the other intermediate frequency transformer 460' is connected" into circuit. The primary winding 46l together with its trimme'r capacitor 464 are now connected in shunt to the fixed loading capacitor 465. The secondary winding 462 and its trimmer capacitor 463 are connected in parallel with the fixed loading capacitor- 4661 The secondary windings 4'62 and 412" 'areeach provided with a midtap which is connected to a conductor 482, whereby the unidirectional current produced by the full wave rectifier action of the diode elements of the vacuum tube 418 istransmitted through a suitable filter to a stage of direct current amplification; order that the response characteristics of the intermediate frequency transformers 460 and 410 maybe broadened further for the transmission of" wide band or frequency modulated sigrials-there are providedresistors 416 and Hi arranged to be connected by switches S56 and S51 respectively, across the primary and secondary windings of the. intermediate frequency transfor-merthen in use.

The vacuumtube 41B is-of the type-havin two diode elements and a-triode element. The cathode of the vacuum tube is connected to ground. The grid of the vacuum tube 418 is connected to an isolating capacitor 5Z6 which is connected to one terminal. ofa fixed padding. capacitor 596 and one contact of each: of the switches S34 and S35. 'Ihe'anode'offthevacuum tube M8 is connecte'd t'o the conductordB I. which is connected to one terminar of a blocking capacitor 591, the otheritenmina-l: of capacitor 506, and one contact of a switch S36.- The'capacitor' 513! is connected. in a series circuit extending from the conductor 48'! through avariabletuning; capacitor 5i32to ground toprevent. the; application of high. voltage to: the lattercapaci-tor. At the. extreme ooun't'ereclocb wise-rpositionzof the rotor of. the capacitor 502 a switch5fl3i'sactuatedto the open circuit position shown. The" switch: 563 is connected in. series between the high. voltage conductor I34 and: a resistor 5I34. which is connected to the midpoints ofatuned inductors 557 and 509. The juncture betweerr the resistor 594 and the-tuned inductors 5.91? and 508 is connected to a grounded by-pass capacitor 505'; The inductor 5&3! is-providedwith arradjustablecapacitor 50B, and similarly the inductor 5.09 is provided. with an adiustablecapacitor-sill. These. capacitors 5638: and 5H operatein parallel. with: the loading; capacitor 506. The

switch. S34 normally is arranged to-short circuit the inductor 561. The switch S35-is. arranged to normally short circuit the other inductor 509. Normally the upper terminal. of the' inductor 501 is connected through the switch S36 to the con:- ductor 481 which is connected to the anode of the vacuum tube 418. In order to determine the frequency of the oscillator for the first range-of signal frequencies being received the switch S34 is. actuated to place into operation the resonant circuit comprising the inductor 501, the parallel capacitors 508 and. 506, andthe series connected capacitors 50! and 562. In order to provide. the proper frequency of oscillation for the upper range of signal: frequencies to be received the switch S34 remains in the position shown in the circuit and the switches S35. and S36 are actuated to place the other resonant circuitinto operation. Energy from the oscillating circuit is transmittedthrough a resistor network including the resistors 461 and see connected. to conductor etifiiand a grounded resistor 168 connected to the-common juncture of the resistors so"; and4'69. The oscillator. is placed into operation byrotation of the rotor of the capacitor 552 which closes the switch'563 thereby to' supply anode energy to a circuit including the resistor 58 1", a portion of the inductor 5M, and the. conductor 48!; The frequency of the oscillatorwithin certain predetermined limits is varied by operation of the variable capacitor 502.

The direct current'produced by the diode sectioniof the vacuum tube 418 is transmitted by the conductor sac through a resistor filter network includin the series connected resistors 512 and 513. The juncture between the first resistor 512 andthe conductor 462' is connected to a grounded capacitor 515. The juncture'between the -two-resistors is connected to grounded capacitor 516. The: remaining terminal of the resistor 513 is conn'ectedto a grounded capacitor 5 i i. The juncture between the resistor 513 and the capacitor 51! is'connected' directly to the grid of a direct current. amplifier vacuum tube 5'! and to a' grid cir cuitcoupling resistor out which is grounded. The cathode of the vacuum: tube 518 is connected through resistors 5 M and 52'! to the cathode filament 518B; one side of which is grounded. The cathode of the vacuum tube iilti'sby-passed'to ground by an electrolytic capacitor 526. A zerocenter direct. current indicating or tuning meter 52-!" connectedin parallel to a resistor 522 and in series with'a resistor 523 is arranged across the resistor? 51 9. In order to vary the sensitivity of the meter 521 there is provided a switch 525 arranged to short circuit the series resistor 523. In order that any radio frequency energy which might ent'erthrough the glass on the face of the instrument 52 which is mounted on the front'of the completely'shielde'd cabinet. might not be permittedstopass't'o otherfioarts of thereceiver; each side-of. the meter 52! is lay-passed: to ground by one-10f two grouhdedl'capacitors524 and 521. The anode of the vacuum tube 51-8 is supplied with voltage from the conductor I34 through the-series resistorstzai and 529, the common juncture of which: is b'y passed' to ground through the grounded capacitor 530'.

In o'rdertoprovide for a test or the operativeness of the visual' indicating channel there is provided. a coupling'capacitor 59!- connected betweenutheanode of the vacuum tube 518' andv a jack 592- which is connectedto ground. A plug and head phones may be connected to the jack in order to=test th'eureceiver and this channel to determine. whether the: operation is proper tacts 551 of a relay 550.

The anode of the vacuum tube 5l8 is also connected to the conductor 53| which is connected to the switch contacts H5 and H6 of the motor driven switch I ID. The remaining contacts of the pairs of switch contacts H5 and I [6 are connected to the conductors 532 and 533 respectively, which are connected through radio frequency choke coils 531 and 538 to opposite terminals of the zero center direct current indicating instrument 539. A pair of capacitors 534 and 535 are connected between'the conductors 532 and 533,. and their common juncture is grounded.

Since only one of these capacitors at any one time. is connected between ground and the conductors 531, only the connected capacitor is therefore charged by the direct current supplied to the conductors 53L The operation of the switch contacts H5 and H6 is concomitant with the periodic reversal of phase of one of the radio frequency energies being combined at the input of the receiver by virtue of the operation of the double pole double throw portion of the motor driven switch H0. Thus when the combinationof energies of the loop antennas I01 and I92 together with the sense antenna N13 is transmitted through the receiver, converted by the diode portion of the vacuum tube 418 and amplified by the vacuum tube 518, this direct current is supplied to one of the capacitors534- or 535. When, however, the energies from the spaced parallel opposed antennas and the sense antenna are combined with reverse phase relation the corresponding direct current produced by the vacuum tube '418 and amplified by the vacuum tube 5I8 is supplied to the other of the two capacitors. The meter 539 is responsive tothe difference in voltage between the capacitors 534 and 535. In order that the meter 539 give a steady indication, or in other words be deadbeat, there is provided an alternating current electrolytic capacitor 536 connected between the conductors 532 and 533. A resistor 543 is connected in parallel to the alternating current electrolytic capacitor 536 so as to control the sensitivity of the indicating meter 539 and to provide a discharge circuit for the capacitors. The sensitivity of the meter 539 furthermore may be varied by connecting a resistor 544 in parallel with the resistor 543 by operation of a switch 546. The radio frequency choke coils 531 and 538 serve to prevent any radio frequency energy from being fed back into the receiver which might enter into the indicating instrument 539 through the glass dial, since the instrument is mounted in the front of an otherwise completely shielded cabinets In order to dissipate such radio frequency energy each terminal of the indicating instrument 539 is by-passedto ground by grounded capacitors 54! and 542.

The entire radio receiver is energized from two six-volt storage batteries 541 connected in series.

One terminal of these batteries is connected through filter choke coils 552 and 553 to a power switch 558. The other side of the power switch 558 is connected to a conductor 5| 0 which extends to a number of filaments of the vacuum tubes.

The same terminal of the storage batteries 541 is also connected to one contact of a pair of con- The relay 550 is connected in series between the other terminal of thebatteries 541 and a plurality of choke coils' 548 and 549 which are connected to conductor 50!) pair of capacitors 554 and 555 arranged in'series is connected across the relay 550 and the batteries 541. The common juncture of these capacitors is connected to ground. Between the common juncture of the choke coils'552 and 553 and the choke coils 548 and 549 also are connected a pair of series-connected capacitors 556 and 551, the common juncture of which also is grounded. One side of the power switch 558 is connected to a conductor 561 which, in turn, is connected to a grounded capacitor 559. The common juncture between the choke inductors 552and 553 is connected to a conductor 516. The conductors 56I and 510 are provided for the energization of such auxiliary equipment as may be needed in connection with the. operation of the direction finder. Such equipment, for example, may comprise a remote indicating or telemetric system for indicating at a remote point the azimuth of the directional antennas HH and I62; Such apparatus, for example, is shown, claimed and described in the copending applications of Edward S. Peterson, Serial No. 501,956, filed September 11, 1943, now Patent No. 2,411,389, granted November 19, 1946, and Serial No. 501,957, filed September 11, 1932, now Patent No. 2,427,621, granted September 16, 1947. When the power switch 558 is closed, energy flows through the relay 55!! causing it to close its contacts 55L This completes a circuit from the storage batteries 541 to a plate voltage supply apparatus 560. The output of the plate voltage power supply 560, which may be of any desired type including a vibrator rectifier power pack, is transmitted througha filter circuit including the series choke coils 562 and 563 and grounded filter capacitors 564 and 565 to the high voltage conductor I34. 7

At the same time that the switch 558 is closed energy now flows out over the conductors 500 and 5H] to supply energy to the filaments of the various vacuum tubesand to certain dial lights. For example, the conductor 5l0 may be connected to one side of a plurality of filaments such as ZMF, 39IF, 45IF, 418F, 6|9F, 643F and 19F. One side of each of these filaments is connected to ground and the other side of each of these filaments is suitably by-passed to ground by capacitors 516, 511, 518, 519, 58!, 582 and 583, respectively. The conductor 500 is connected to the filaments of various vacuum tubes such as the filaments I28F, 326E, 426F,, 5|8F and 6|JIF' which have one side thereof connected to ground. The other side of each of these filaments is suitably by-passed to ground through capacitors 51l to 515, respectively. 'A plurality of dial lights 566 to 568 are also connected between ground and the conductor 500. In order that the currents supplied by both conductors 500 and 5H] might be equal a balancing resistor 569 is connected between ground and the conductor500. It will be noted that the batteries 541 have a common juncture which is not directly connected to ground but which is effectively connected to ground through the grounded sides of the various filaments associated with the power conductors 500 and 5I0. It furthermore will be remembered that the cathode circuit of the vacuum tube 5|8 includes the filament 5l8F which is grounded, so that a six volt positive bias is supplied to the grid circuit of the direct current amplifier tube 5l8. This'has the effect of placing weak signal direct current voltages of the full wave detector 418 on the linear portion of the tube characteristic of the direct current amplifier tube 5l8.' Thus correct proportional amplification is provided for weak signals so as to avoid errors which otherwise might be intro duced due to tube noises because of thehigh 17 overall gain of the radio receiver required due to the low energy level obtained by the use of loop antennas.

The intermediate frequency amplifier transformer 430 or 440 is capacitively coupled by the capacitor 484 to the conductor 483 which extends to the control grid of an intermediate frequency amplifying tube Bill Normally the vacuum tube Elli operates as an intermediate frequency amplifying tube in the audible signal channel portion of the radio receiver. The cathode of the vacuum tube 6M is connected to a grounded bypass capacitor 602 and through the normally closed switch S43 to a resistor 603 which is connected to the conductor 329 which extends to the resistor 454 to ground. Since the resistor 454 is arranged to be short clrcuited by operation of the switch S52, it becomes apparent that the bias on the vacuum tube Bill is also changed at the same time that the bias on the intermediate frequency amplifier tubes 326, 426 and 45! is changed. When the audible signal channel is to receive frequency modulated signals the operation of the vacuum tube Bill is changed to a frequency modulation limiter. This changeover is accomplished by operation of switches S42 and S43, the latter switch therefor operating to connect the cathode of the vacuum tube 6B! directly to ground when it is to operate as a frequency modulation limiter.

The vacuum tube 60! is arranged to operate at either of the two intermediate frequencies at which the first intermediate frequency amplifying tubes operate, and for this purpose there are provided two intermediate frequency transformers 6H) and $20. Normally the one terminal of the primary winding 62! of the intermediate frequency transformer 620 is connected through the switch S31 to, the anode of the vacuum tube 60!. The other terminal of the primary winding 62| of the transformer 620 is connected through a voltage drop resistor 604 to the high voltage conductor l34. The connection between the resistor 694 and the primary winding 62! is also connected to a grounded by-pass capacitor 605. The primary winding 62! is arranged to be tuned by an adjustable capacitor 623 arranged to operate in parallel with loading capacitor 601. The secondary winding 622 of the transformer 628 is normally connected through switches S38 and S39 to the rectifier plates of a full wave restifier tube M9. The secondary winding 622 of the transformer 628 is arranged to be tuned by an adjustable capacitor 624 operating in parallel with another loading capacitor 609. In order to broaden the response characteristic of the transformers 6H] and 620 there is provided for the primary windings a resistor 606 connected in parallel to the loading capacitor 607, and for the secondary windings, a resistor 608 connected in parallel with the loading capacitor 649. The secondary winding 622 of the transformer 620 is provided with a midtap which for amplitude modulated, and continuous wave signals is connected by a switch S40 to a conductor 632. The midpoint of this intermediate frequency transformer secondary winding 622 is also connected to a phasing capacitor 6l8 which is connected to the anode of the vacuum tube Bill for operation when frequency modulated signals are being received. For the reception of frequency modulated signals the switch S40 joins the midpoint of the intermediate frequency transformer winding 622 to the conductor 63L For operation at the other intermediate frequency, the transformer 6"] has a primary winding s! l' arranged to be tuned by an adjustable capacitor M3 arranged to operate in parallel with the loading capacitor Gill. The secondary Winding (N2 of the transformer Bill is tuned by an adjustable capacitor 6E4 arranged to operate in parallel with the loading capacitor 609. The resistors tilt and 608 are arranged for broadening the response of either of the transformers cm or 628, dependent 'upon the condition of operation of the switches S31 to S39. The midpoint of the secondary winding BIZ of the intermediate frequency transformer cm is connected to the switch S40 and to the phasing capacitor 6E8.

When the switches Set to S43, inclusive, are in the positions shown, the vacuum tube GUI operates as a final stage of intermediate frequency amplification which has a broader response characteristic than the previous stages of intermediate frequency amplification. Suitable screen grid voltage is supplied from the Voltage conductor use through the resistor 694 and another resistor 6I5 which is connected to the screen grid of the vacuum tube Hill. The screen grid of the vacuum tube to! is also connected to a grounded by-pass capacitor GIG and to the switch S42. The switch S42 is connected through a resistor 6H to ground. When the operation of the vacuum tube 60! is altered for the reception of frequency modulated signals the switch S43 grounds the cathode of the vacuum tube, thereby to remove the bias heretofore appearing in. the grid to cathode circuit of the tube. At the same time the potential applied to the screen grid of this vacuum tube till is altered by the closing of the switch S42, which now forms a voltage divider circuit between ground and the voltage conductor i341 and which includes the resistors Bil, M5 and 684 in series. It is believed that such change in the operation of the vacuum tube Elli will be readily understood by those skilled in the art as the operation in. the form of a limiter corresponds to the operation already known in the art.

The outputs of the intermediate frequency transformers 6i!) or 620 are arranged to energize the plate electrodes of the full wave rectifying tube (H9. The cathodes of the vacuum tube (H9 are connected to ground, one directly to ground and the other by the operation. of the switch S4! for the reception of continuous wave or amplitude modulated signals. Under such operation the audible signal components produced by the vacuum tube 6H! appear in the secondary windings of the intermediate frequency transformers which, it will be recalled, are normally connected through the switch $46 to the conductor 632, which is connected to a resistor capacitor filter network comprising the resistors 524, 635 and '636, the latter resistor being provided with an ad ustable contact 631'. The junctures between the conductor 632 and the various resistors are each connected to one of a plurality of grounded capacitors 638, 639 and MI. The adjustable contact '53! on the resistor 636 is connected to a coupl ng capacitor sea which is connected to the grid of the audio ampl fier tube 643. The grid of this vacuum tube is provided with a grounded rid resistor 644. The cathode of the vacuum tube 643 is suitably b ased by a grounded resistor 645 which is by-passed by a capacitor 545. The output circuit of the vacuum tube 643 includes a coupling resistor 64? connected be- .19 tween the anode and the. voltagedrop resistor i548. fl he uncture between these two resistors ,is connected to a grounded by-pass capacitor 549. .The voltage drop resistor-MB is connected to the high voitageconductor I34.

the grounded jack 552. provided tor the reception of a phone plugconnected to a pair of head I phones.

In order to receive frequency modulated waves there is provided a combination. of resistors and capacitors connected between the two cathodes the resistor 6'25 and another capacitor 628 is connected across the resistor 626. One cathode of the vacuum tube (H9 is connected directly to ground. The other cathode of the-vacuum tube 619 is connected to the conductor 633 so that when the switch S4! is in the position shown it is connected to ground, but when the switch S4,!

is actuated for the reception of frequency modulated waves the conductor 633 is connected to the .conductor 632 which, in turn, is connected to the resistance capacitance filter arrangement preceding the input to the amplifying tube 643.

The anode .of the vacuum tube 643 is connected to a blocking capacitor 65! which, inturn, is connected to For frequency modulation reception, therefore, I

the midpoint of the secondary winding of the intermediate frequency transformer is connected to the resistor 629. 'Due to the change in the connections to thecathodes of the vacuum tube GIS when the switches S40 andSM are operated, the two rectifier portions .of the vacuum' tube 619 function as differentially arranged detectors operated from a single intermediate frequency amplifier or limiter. Since this circuit operation corresponds to discriminators or frequency detectors already known in the art no further explanation of the operation thereof is deemed to .be necessary,

In a single vertical loop antenna a vertically polarized wave traveling horizontally and parallel with the plane of the loop induces therein a voltage which appears at the terminals of the loop. As the loop is rotated with respect to such horizontally traveling vertically polarized waves the voltage appearing at the terminals of the loop will decrease until the plane of the loop is perpendicular to the direction of 'travel of such Waves. As the loop then is at right angles to the wave no voltage appears at the output terminals. The loop antenna may then continue to be rotated until it has been moved through an arc of one hundred and eighty degrees, whereupon a maximum amount of voltage will again appear at the output terminals. A further rotation of the loop through ninety degrees reduces the output voltage to zero and a subsequent rotation again increases the output voltage to a maximum. Direction flndingloop antennas are usually provided with an azimuth scale of three hundred and sixty degrees and the voltage res onse obta ned by the rotation of the loo antenna with res ect to vertically olarized horizontallv travel n waves mav be lott d in p ar co-ordina es so that the resultant vo ta e. represented w th res ect to the angular displacement or azimuth of the 1000 forms a figure ei ht pattern, such as shown in Fig. '7. The voltage obloop antenna.

tained whenthe loop is at the ninety degreepositionof Big. 7' is maximum and when the loop is at two hundred and seventy degrees it is-also maximum but of opposite phase, and hence it has been common to represent one of the circles of the figure eight as positive and the other circle as negative to indicate this phase change.

The voltages induced in-the loop antenna, however, may also be represented by linear coordinates as shown in- Fig. 8. The single loop antenna shows the greatest change in induced voltage at the zero and one hundred eighty degree positions of Figs. '7 and 8 and hence these points are better-suited for determining the direction of a radio transmitter than would be the maximum voltage pointsat ninetydegrees or at two hundred and seventy degrees. The minimum points are therefore known as trueand reciprocal null points. A single loop antenna, however, is subject to a distortion or displacement of the null points by-horizontally polarized wavestraveling in a non-horizontal direction as is the case of a wave reflected from the Kennelly- Heaviside layer, which error has been called the night effect. A single figure loop, therefore, indicates the plane in which the transmitter and the receiver are located but does not indicate the direction from the receiver to the transmitter and, hence, this is termed to be a one hundred and eighty degree ambiguity of this directional antenna system.

ln'order .to avoid one hundred and eighty degree ambiguity it has been proposed to combine response pattern suchas the cardioid shown :in Fig. 9 which has but one null position.

The cardioid of Fig. 9, however, still has anerroneous null position due to the night effect upon the The response of such antenna system may also be plotted in linear co-ordinates as shown in Fig. 10.

In order to obviate night efiect directional antenna systems such as the Adcocl: system or the spaced loop antenna system have been suggested. The response pattern of such antenna systems is commonly termed a clover-leaf :and this has been shown in Fig. 11. Translated into linear coordinates, the representation would appear as a sinusoidal curve shown in Fig. 12. The spaced loop system, however, while having the advantage of the elimination of night effect has the great disadvantage of having four null positions which are referred to as ninety degree ambiguities.

In order to obtain the benefits of a system wherein the night effect has been eliminated, the present invention combines the output of a spaced loop antenna arrangement with another loop antenna known'as a sense loop antenna. The spaced loop antenna has a response characteristic such as shown in Fig. 12 and this is also shown by the curve A of Fig. 13. The volt- ,age of the spaced loop antennas shown by the curve-A is combined with the volta e obtained from the single intermediate loop ant nna which is shown by the curve B which is similar to the curve shown in- Fig. 8 but displaced by an angle of ninety degrees.

In accordance with the present invention the voltages obtained from the two antenna systems, and corresponding to the curves A and B, are

periodically displaced in phase by one hundred and eighty degrees. This is obtained by operation of the motor driven switch IIU. When this is accomplished by the switch the resultant of the curves A and B will appear as curve D in Fig. 14. In the visual signal channel portion of the receiver the full wave diode portion of the vacuum tube 418 responds to the resultant curves C and D to produce uni-directional current. If the curve D, for example, is rectified there will then appear a curve such as E. Uni-directional currents represented by the curve E, also shown in Fig. 15, are transmitted to one of the two capacitors 53 i or 535 dependent upon the position of the switch contacts I 15 or I 16. It may be assumed that the direct currents corresponding to the curve E are supplied to the capacitor 534. When the alternating currents corresponding to the curve C of Fig. 13 are rectified they will produce uni-directional currents corresponding to the curve F of Fig. 15. These uni-directional currents are supplied to the capacitor 535. The two curves as represented in Fig. 15, therefore, portray the instantaneous voltage charges upon the capacitors 534 and 535 as related to the azimuth or angular displacement of the directional antenna system. The two capacitors are arranged in opposition and the zero center direct current instrument 539 responds to the resultant voltage of these two capacitors. The resultant voltage is therefore represented by the curve G; It therefore will be noted that at zero azimuth of the loop antenna system the resultant voltage is zero, so that the pointer of the indicating meter remains at the center.

The pointer of the zero center indicating instrument, in accordance with rotation of the loop antenna, therefore moves to one side of the zero point in accordance with the first half-wave loop of the curve G, and at some point intermediate zero and one hundred and eighty degrees again returns to zero. Thereupon, with further rotation of the antennas the second half-wave loop of the curve G is followed by the instrument and at one hundred and eighty degrees rotation of the antenna zero current is indicated and further rotation produces a movement of the indicator needle to the other side of zero to follow the third half-wave loop of the curve G, and again the meter may return to zero at an intermediate point between one hundred and eighty degrees of the voltages combined, or in other words the amplitudes of the curves A and B, may be varied considerably but the resultant curve C will always have a similar shape although of difierent magnitude. When the resultant voltages are translated into direct currents the curves E and F accordingly will vary to slightly change the resultant curve G in amplitude and in the shape of the half-wave loops. The cross-over points at zero and one hundred and eighty degrees, however, remain constant.

With certain ratios between the voltages represented by the curves A and B of Fig. 13, the resultant curve C when rectified may produce a resultant direct current curve such as G where, at the intermediate points between zero and one hundred and eighty degrees, and one hundred and eighty and three hundred and sixty degrees, the meter needle may not quite return to zero or may go slightly beyondzero and back again. Such deviations from the zero line of the representation in Fig. 15, however, are readily observable and the rate of change of the movement of the needle is difierent at these points than at the true and reciprocal nullpositions corresponding to zero and one hundred and eighty degrees.

Due to the opposed connection of the spaced loop antennas the greatest rate of change of indicator response per degree of loop antenna rotation occurs at the time when each loop antenna is responding to the maximum to the desired signal. At that time each loop antenna is in a plane parallel to the plane common to the radio transmitter and the radio receiver. In such position each of the loop antennas has a maximum response to vertically polarized waves, irrespective of the angle of elevation of the direction of travel of such waves, and each antenna has a minimum response to non-horizontally traveling horizontally polarized waves. Therefore, the antenna system thus provided is free from night effect.

The ideal spacing between the spaced loop antennas is one which delivers the most rapid rise in indicator response per degree of loop antenna rotation, which would be one-quarter wave length for any particular signal frequency. At greater values of spacing the indicator response is less desirable. For any practical portable direction finding equipment covering a range of signal frequencies, the loop antenna system is a compromise in which the considerations are the receiver sensitivity at the various frequencies, noise level at various frequencies, and such practical considerations as the length of arm which can be readily transported. Because of these various considerations it has been found desirable in the present arrangement to provide a spacing between the loop antennas WI and 102 which is in theorder of one-quarter wave length, for the highest signal frequency to be received, and consequently this spacing value is a smaller fraction of a Wave length for any lesser frequencies. Thus for all frequencies to be received there is obtained the more desirable response per degree of loop antenna rotation.

The radio receiver, the circuit of which is schematically shown in Figs. 1 to 6, is provided with a band switch mechanism which comprises a shaft having thereon numerous cams each arranged at various angular displacements to actuate certain switches shown in Figs. 1 to 6. .Reference may now be had to Fig. 16 wherein there is shown a table indicating which switches are actuated in accordance with the rotation of the band switch mechanism to the various positions, which are ten in number. The first two positions cover the first part of the lower frequency range as, for example, from 2 to 3 megacycles. Position No, 1 conditions the receiver for the reception of narrow band modulated or continous wave signals, and position No. 2 conditions the receiver for the reception of wide band modulated signals, such as frequency modulated signals for which the receiver response characteristic is permitted a deviation of plus or minus 15 kilocycles. The band switch position No. 3, which covers from 3 to 4.5 megacycles, is provided for the reception of continuous wave and amplitude modulated signals similar to the operation in band switch No. 1. Band switch No. 4 broadens the response of the receiver to a deviation of plus or minus 15 kilocycles similar to the broadening obtained in band switch No. 2.

The higher frequency range is also divided into 

